Unusual defect and domain structure in YBa₂Cu₄O₈ (Y124) single crystals

Bulk single crystals of YBa₂Cu₄O₈ (Y124) have been investigated using conventional, high resolution and analytical electron microscopy techniques. Complex and profuse dislocation structures are observed in the as-grown crystals. At least two different types of dislocations are identified, with two different Burgers vectors and slip planes. The analysis of the dislocation geometry strongly suggests that they are (or were) probably quite mobile, at least at some elevated temperature. An unusual domain structure contrast is also observed in some areas. The local chemistry and crystallography of such regions appear to be close to that of Y124. EELS O K core loss analysis indicates that such regions are devoid of holes as reflected in the lack of a pre-edge peak in O K EELS spectra. These observations are discussed in terms of the influence of microstructural details on the physical properties of Y124.